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xc71: that seems a bit odd. Why would a 27.2mm post need less insertion than a 31.6mm post (and taken to the extreme it clearly seems nonsensical)?

As others have said the min-insertion should be the larger of the seatpost's min-insertion, and what the frame requires (normally bottom of post passing the toptube-seattube junction, but in some cases like Ti frames where there's a seatpost insert, basically just reaching to the bottom of that insert since anything below that is just hanging in space).

xc71: that seems a bit odd. Why would a 27.2mm post need less insertion than a 31.6mm post (and taken to the extreme it clearly seems nonsensical)?

As others have said the min-insertion should be the larger of the seatpost's min-insertion, and what the frame requires (normally bottom of post passing the toptube-seattube junction, but in some cases like Ti frames where there's a seatpost insert, basically just reaching to the bottom of that insert since anything below that is just hanging in space).

Got this info straight from one of the owners of Ibis bikes. It is the exact formula they use for their frames/seatposts.

Such a formula might have little relevance to other frames though which the OP should be aware of. Frames which have a brace between the top tube and seattube often require longer insertion for example. As before, the best thing to do is check with the frame manufacturer.

I suspect he was either a) feeding you a line to shut you up or b) dressing a vague rule of thumb up as a calculation based on mechanical principles.

Cos 2.5 times diameter would only apply to one specific design of frame, in one particular material. Unless he was just talking about the seatpin (not the frame at all) which is a bit daft, as the insertion requirements for the frame take precedent.

You may well find it is just a rule of thumb minimum, i.e. 2.5 times diameter rounded up to the nearest whole inch. A lot of "design rules" eventually come down to this once you ask the right questions.

I suspect he was either a) feeding you a line to shut you up or b) dressing a vague rule of thumb up as a calculation based on mechanical principles.

Cos 2.5 times diameter would only apply to one specific design of frame, in one particular material. Unless he was just talking about the seatpin (not the frame at all) which is a bit daft, as the insertion requirements for the frame take precedent.

You may well find it is just a rule of thumb minimum, i.e. 2.5 times diameter rounded up to the nearest whole inch. A lot of "design rules" eventually come down to this once you ask the right questions.

Too funny, Ibis had also posted the same info on their Forum for other customers inquiring about seatpost insertion.At any rate Jacob, your frame manucturer will give you the min. insertion number. I'm 1mm difference than your buddy's suggestion if you have a 31.6mm seatpost.

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